What will infantry school be like as a prior service SGT from the marines that already has a combat deployment under their belt goin army ?

Good advice from others that all students are equal. Do not bring up your past experience to anyone and you'll have an easier time completing the school.

Bored

SFC (Join to see) gave, as usual, sound advice. Beyond that, when I went Army from the Navy, back then Navy and USAF prior service HAD to do Army Basic. "Infantry" school may be different for you. One of my sons, prior Army Reserve did his Basic at Ft. jackson, SC. When he went Regular Army, he opted for Infantry and Airborne, and RIP (Ranger wannabe). He did have to do "Infantry" school which is both Army Basic Combat Training, AND Infantry, or emphasis on Infantry. He joined a class that was four weeks from graduation since he already had Basic Training.
Funny thing was, to him it seemed the Infantry Basic was backwards. In other words, the last four weeks, they covered more of what he already had at Ft. Jackson, and NOT so much focus on Infantry stuff. When he was sort of complaining about that at the time, I gave him SFC (Join to see)'s advice - "Shut mouth ... open ears - you *WILL* ALWAYS learn something." It was almost the motto for fire fighting training where we often took the same class yearly, KNOWING that we'd learn something we didn't catch before.
You may not loose a stripe ... it would be interesting to know.

Are you tactically sound as an infantryman?

A light breeze

I'll give you the same advice that I gave my son when he enlisted: Keep your eyes and ears open and your mouth shut. Your prior service will likely be known by your DIs and they will be testing you. Follow instructions precisely. Don't flaunt your prior service to your fellow trainees (yes you are a trainee). Speak only when addressed by your drill instructor. You may have to bite your tongue when you disagree with your trainers but do it (remember you are being tested). Best of luck to you

I'll give you the same advice I gave my son when he enlisted. "Keep your eyes and ears open and your mouth shut." For you there will be several advantages here: The fact that you are prior service will be known by your DIs ; Try to not draw negative attention to yourself by flaunting your experience, maintain a low profile. Try to follow instructions precisely, this should not be a problem. Speak only when spoken to. Best of fortune to you.

Instead of getting tons of unverified opinions here, go talk to your local Army Recruiter. They have access to the Army Regulations for Enlistment. They can give you the straight facts, and not opinions. They can look up to see if you’re even qualified to enlist in the Army. You’ll need to bring your DD-214 with you so they can look up your discharge code. Have you had any field grade UCMJ action? Were you discharged honorably? How long have you been out of the Marines? Do you have any civilian misdemeanor or felony charges, whether completed or open? Those are some of the things that they will ask you. And, depending on how long ago you were discharged as a Sgt, may mean you may/may not retain that rank if you are eligible to enlist. The other factor is if the Army is accepting Prior Service enlistments. Do you have any Military Service Obligation left?

When I was an Army Recruiter from 1989-1992, the Army wasn’t accepting prior service. That may have changed now.

But, before anyone can answer your questions, you must first go see your local recruiter to see if you’re even eligible and whether or not the Army is accepting prior service. Without those steps, the answers won’t matter.

Although you'll have to go through recruit training again, I don't think you should be treated like some dude, however, you may have to prove yourself. In my opinion, Sgt and a GySgt are the two most influential leadership ranks who also get shit done. Once people know your history, I think they'll give you the respect you deserve. Do not going expecting people to bow down, nor with a "chip on your shoulder." Your prior military experience, leadership skills, character, and attitude will "speak for itself," they will come in handy when the time is right. Semper Fi!

May want to look at it as an opportunity to both learn how the Army does things, AND as a leadership opportunity to help out some of the younger troops who may be having difficulties, with "school stuff" OR Life in General stuff. The Life in General things are, in my experience, the reason why many troops end up leaving well before their normal ETS; drugs, DUI, AWOL, running off at the mouth, etc. You CAN make a difference here.